The primary objectives of the proposed research are (1) to analyze the multi-locus genetic structure of natural populations of Salmonella and certain other human-pathogenic bacteria through the application of several methods of indirectly and directly detecting nucleotide substitutions in chromosomal genes, combined with the mathematical theory and statistics of molecular population genetics; (2) to develop basic evolutionary genetic frameworks within which to analyze and interpret data on the distribution of cell-surface antigens, plasmids, and other characters that have been implicated as determinants of pathogenicity and disease specificity; and (3) to develop powerful new genetic markers for epidemiological studies of Salmonella and other important human pathogenic bacteria. Proposed studies include: (A) Continuation of an analysis of electrophoretically demonstrable allelic variation in 23 chromosomal enzyme genes to assess clonal diversity and relationships in the medically and economically most important serotypes of Salmonella, with special reference to variation in antigenic profile and the evolutionary origins of typhi and other organisms causing human typhoid fever. (B) Determination of the genetic diversity, genomic relatedness, and extent of clonal host specificity among strains of serotypes that are commonly recovered from both humans and domesticated animals. (C) Analysis, by nucleotide sequencing, of the molecular and population genetic bases for the occurrence of the same somatic (O) and flagellar (H) cell-surface antigen profile in strains belonging to distantly related phylogenetic lineages. (D) Development of a molecular epidemiological marker system for discrimination among isolates of the predominant clones of the Salmonella serotypes enteritidis and typhimurium. (E) Selection and characterization of a standard set of Salmonella reference strains (SARC) that are representative of the full span of genotypic diversity within the genus for general use in microbiological research. Other research will be concerned with the molecular genetic correlates of phylogenetic differentiation of populations of Escherichia and Salmonella.